Why is x their common multipler

The variable $x$ represents this common unit or multiplier. If $x$ is 1 year, the ages are 3, 4, and 5 years. If $x$ is 10 years, the ages are 30, 40, and 50 years. By using $3x$, $4x$, and $5x$, we ensure that the ages maintain the given ratio while allowing us to find their actual values.